Frame recursive filters have been found to be effective in improving the signal-to-noise ratio of video signals having little or no frame-to-frame motion. Such filters are based on the principle that for still images, or those having little motion, there is a high correlation of picture element values from frame-to-frame whereas the noise (for still images) is relatively incoherent and exhibits little correlation from frame to frame. Accordingly, by combining a number of frame delayed signals, the signal power of the sum will increase more quickly than the noise power and the signal-to-noise ratio will increase.
Obtaining multiple frame delayed video signals may be achieved by feedback in so-called "recursive" noise reduction systems. FIG. 1 herein is an example of such a system. The noise reduction system 100 of FIG. 1 includes an input terminal 102 for receiving a video input signal to be noise reduced and an output 104 for providing a noise reduced video output signal. Input 102 is coupled via a subtractor 106, a limiter 108, an attenuator 110 and an adder 112 to the output 104. The input 102 is also applied to the adder 112 and the adder output is feedback to the subtractor 106 via a frame delay unit 114.
In operation, a noisy video input signal S1 (at terminal 102) and a frame delayed noise-reduced video output signal S3 from unit 114 are combined in subtractor 106 to produce a frame difference signal S4. The frame difference signal S4 provides an estimate of the amount of noise in the input signal S1 for still images. For moving images, it represents frame-to-frame motion plus noise. To reduce the visual artifacts produced for moving pictures to an "acceptable" minimum level, difference signal S4 is limited in limiter 108 to relatively low levels (e.g., a few IRE units). Providing a larger limiting threshold will increase the effective noise reduction but increase the artifacts caused by scene motion. Accordingly, limiting values are generally selected as a compromise between the desired noise reduction and the acceptable level of motion artifacts.
The output of the limiter 108 is then reduced (attenuated) in attenuator 110 by some small amount (a reduction of 1/8th or a transmission of 7/8ths) so that transients of the system will eventually decay to zero. Advantageously, the attenuation ensures convergence and stability of the feedback system. Finally, the reduced output of the limiter S5 is attenuated in attenuator 110 and is added, as signal S6 in adder 112, back to the current input signal S1 thus canceling a portion of the noise content of the input signal S1. The resultant "noise reduced" signal S2 is fed back to the frame delay to be used on the next frame and thus completing the recursive signal path.
As described above, the general form of recursive noise reduction provides excellent results for still (or nearly stationary) images because of the high correlation (coherence) of picture element values from frame to frame whereas the noise is relatively uncorrelated on a frame to frame basis. For moving pictures, however, the difference signal S4 represents motion as well as noise and limiting at a relatively low level is required to minimize the visual artifacts caused by the motion component of the difference signal. Generally speaking, however, in certain applications limiting the difference signal in a frame recursive noise reduction system may not be enough to suppress the motion artifacts to an acceptable minimum level and additional motion compensation may be called for and various approaches to this problem have been proposed.
One way to reduce the effect of motion artifacts in a frame recurslye noise reduction system is to split the video band into two portions, apply frame recursive noise reduction to the lower frequency portion and apply another form of noise reduction to the higher frequency portion. Such a system is described by Christopher in U.S. Pat. No. 5,130,798 entitled DUAL BAND PROGRESSIVE TELEVISION SYSTEM WITH NOISE REDUCTION, which issued Jul. 14, 1992.
Advantageously, the "dual band" approach both reduces the system memory requirements and confines transient artifacts to low frequency luminance signals only. On the other hand, relatively complex circuitry is required for the band splitting and separate noise reduction processing functions.
Another approach to reducing the effect of motion artifacts in a frame recursive noise reduction system is described by Takahashi in U.S. Pat. No. 4,246,610 entitled NOISE REDUCTION SYSTEM FOR COLOR TELEVISION SIGNAL, which issued Jan. 20, 1981. The Takahashi approach includes bypassing the frame recursive filter for scenes containing high motion. To this end a motion dependent switch is provided (known generally as a "soft switch") which "blends" the incoming video signal with the noise reduced video signal as a function of the motion indicating signal. For still images the switch selects the noise reduced signal, for moving images the switch selects the original signal (with no noise reduction) and for images having relatively low motion the signals are proportionately "blended" as a function of the motion (frame difference) signal. Although such an approach is effective in reducing motion artifacts in the case of moving images, it also eliminates the noise reduction.
A further problem with the "blending" type of "motion adaptive" noise reduction system is that the action of the soft switch itself may produce visual artifacts for certain scenes due, for example, to motion detection errors. For example, large amounts of noise in a still image may be erroneously interpreted as motion in the scene. If so, the soft switch will erroneously reduce the frame recursive filtering component thus reducing the noise reduction when, in fact, there is no reason to do so.